1. Field of the Invention
The present invention generally relates to marine gyro compasses and, more particularly, is directed to a fast settle apparatus and an error correcting apparatus thereof.
2. Description of the Prior Art
Referring to the drawings in detail, and initially to FIG. 1, let us describe a gyro compass described in Japanese Patent No. 428317 as an example of a conventional gyro compass to which a fast settle apparatus of the present invention can be applied.
The entirety of the gyro compass is depicted by reference symbol A in FIG. 1, and the gyro compass A includes a gyro case 1. As shown in FIG. 1, the gyro case 1 houses therein a gyro rotor (not shown) which is rotated at high speed and at a constant revolution rate by an induction motor (not shown), and a rotary vector of the gyro rotor is directed to the south (i.e., directed in the clockwise direction as viewing from the north). The gyro case 1 has a pair of vertical shafts 2, 2' protruded from the upper and lower portions thereof, and these protruded vertical shafts 2, 2' are respectively fitted into inner rings of ball bearings 4, 4' mounted to corresponding positions of a vertical ring 3 provided outside of the gyro case 1. A suspension wire 5 is secured at its lower end to the upper vertical shaft 2 and the upper end thereof is attached to the vertical ring 3 by means of a suspension wire mount 5'.
According to the above-mentioned arrangement, the weight of the gyro case 1 is not applied to the ball bearings 4, 4' of the vertical shafts 2, 2' as a thrust load but is fully received by the suspension wire 5, thereby friction torque of the above-mentioned ball bearings 4, 4' being reduced considerably. A pair of liquid ballistics 6 are mounted on the east and west of the vertical ring 3 in order to apply a north-seeking torque to the gyro.
As shown in FIG. 2, each of the liquid ballistics 6 is a kind of a communicated tube and is composed of reservoirs 6-1, 6-1' disposed in the north and south of the gyro, liquid 6-2 of high specific gravity substantially filled into these reservoirs 6-1, 6-1' substantially up to the halves thereof, an air tube 6-3 communicating the north and south reservoirs 6-1, 6-1' above and a liquid tube 6-4 communicating the north and south reservoirs 6-1, 6-1' below.
Referring to FIG. 1, it will be seen that a damping weight 7 is mounted on the west side of the gyro case 1 in order to damp the north-seeking movement. As shown in FIG. 1, a primary coil 8-1 of a differential transformer for detecting a declination between the gyro case 1 and the vertical shafts 2, 2' of the vertical ring 3 is attached to the east side of the gyro case 1, and a secondary coil 8-2 of the differential transformer is attached to the opposed position of the vertical ring 3, thereby constituting a follow-up pickup 8. The vertical ring 3 includes a pair of horizontal shafts 9, 9' protruded outwardly from the east and west positions perpendicular to both of the vertical shafts 2, 2' and a gyro spin axis. These horizontal shafts 9, 9' are respectively fitted into inner rings of ball bearings 11, 11' attached to the corresponding positions of a horizontal ring 10 provided outside of the vertical ring 3. The horizontal ring 10 has a pair of gimbal shafts 12, 12' disposed at its positions within the horizontal plane and which are perpendicular to the horizontal shafts 9, 9'. These gimbal shafts 12, 12' are respectively fitted into a pair of gimbal shaft ball bearings 14, 14' attached to a follow-up ring 13 disposed outside of the horizontal ring 10.
As shown in FIG. 1, the follow-up ring 13 has upper and lower follow-up shafts 15, 15' and these follow-up shafts 15, 15' are respectively fitted into follow-up shaft ball bearings 17, 17' disposed at the opposing positions of a binnacle 16.
The upper follow-up shaft 15 has a compass card 18 attached at its shaft end and an azimuthal angle in the bow is read by the cooperation of the compass card 18 and a lubber line 18B secured to the binnacle 16 at the corresponding position in the bow side. An azimuth servo motor 19 is attached to the lower portion of the binnacle 16, the rotary shaft 19A of which is coupled through an azimuth pinion 20 to an azimuth gear 21 located at the lower portion of the follow-up ring 13. An azimuth transmitter 22 is attached to the lower portion of the binnacle 16 and its rotary shaft 22A is meshed with the azimuth gear 21 via a gear system (not shown), whereby an azimuth signal is converted into an electrical signal by the azimuth transmitter 22, which is transmitted to the outside.
The part within the horizontal ring 10, that is, the part including the horizontal ring 10, the vertical ring 3, the gyro case 1 or the like is normally called a gyro sensitive unit. The gyro sensitive unit constructs a vertical physical pendulum around the gimbal shafts 12, 12', whereby the horizontal shafts 9, 9' are constantly kept within the horizontal plane regardless of ship's inclination.
If there is a difference between the azimuth of the gyro case 1 and the azimuth of the vertical ring 3, then such difference is detected and converted into an electrical signal by the follow-up pickup 8 provided between the gyro case 1 and the vertical ring 3. The resultant electrical signal is amplified by an external servo amplifier 23 and supplied to the azimuth servo motor 19 (which forms an azimuth servo system). The rotation of the azimuth servo motor 19 is transmitted through the rotary shaft 19A, the gear train (not shown) and the azimuth gear 21 to the follow-up ring 13 and is further transmitted through the horizontal ring 10, the horizontal shafts 9, 9' or the like to the vertical ring 3, thereby constantly holding the azimuthal error between the vertical ring 3 and the gyro case 1 at zero.
Owing to the action of the azimuth servo system, the horizontal shafts 9, 9' and the gyro spin axis are constantly kept in an orthogonal relation and the gyro can be prevented from being applied with twisting torque. That is, owing to the actions of the three shafts such as the vertical shafts 2, 2', the horizontal shafts 9, 9' and the gimbal shafts 12, 12' having the servo system, the gyro case 1 is completely isolated from the angular motion of the ship, thereby the gyroscope being constructed.
The above-mentioned liquid ballistics 6 are adapted to give the gyroscope the north-seeking force, i.e., function as the compass.
The principle of the liquid ballistic 6 will be described with reference to FIG. 2. FIG. 2 shows the case such that the north-seeking end of the gyro is inclined from the horizontal plane by an anlge .theta.. In this case, assuming that the ship is in its stopped condition, then the liquid surface of the liquid 6-2 becomes perpendicular to the direction of gravity force g. Therefore, as compared with the case such that the inclination of the north-seeking end relative to the horizontal plane is zero, the liquid at the hatched portion of FIG. 2 is decreased in the north-side reservoir 6-1' and is increased in the south-side reservoir 6-1. Assuming now that r.sub.1 is a distance from the horizontal shafts 9, 9' to the center of the two reservoirs 6-1, 6-1', S is a cross section area of the two reservoirs 6-1, 6-1' and .rho. is a specific gravity of the liquid 6-2, then the weight of the liquid at the inclined portion is expressed as: EQU S.times.r.sub.1 sin .theta..times..rho..times.g
Since the above-mentioned weight unbalance occurs in the two south and north reservoirs 6-1, 6-1' and the moment arm from the horizontal shafts 9, 9' is r.sub.1, a torque T.sub.H produced about the horizontal shafts 9, 9' by the liquid ballistics 6 when the north-seeking end of the gyro is inclined from the horizontal plane by .theta. is approximately calculated as: EQU T.sub.H =2 S r.sub.1.sup.2 g .rho..theta. EQU 2Sr.sub.1.sup.2 g.rho.=K
where K is the ballistic constant. That is, the liquid ballistics 6 act to apply the torque proportional to the inclination relative to the horizontal plane of the gyro spin axis to the surrounding of the horizontal shafts 9, 9' of the gyro, thereby rendering the north-seeking force to the gyro. Thus, the gyro is rendered the gyro compass'.
As described above, we have considered so far the case that the ship is in the still condition. In this case, assuming that .alpha..sub.N is a south-north component of ship's acceleration due to increase and decrease of ship's speed, ship's turning or the like, a torque T.sub.H1 generated from the liquid ballistic 6 under the ship's sailing condition is expressed by the following equation: ##EQU1## As shown in FIG. 3, the damping weight 7 is attached to the gyro case 1 with a distance r.sub.2 (in the direction perpendicular to the sheet of drawing) from the vertical shafts 2, 2' within the plane including the vertical shafts 2, 2' and perpendicular to the gyro spin axis. FIG. 3 shows the gyro case 1 under the condition such that the north-seeking side of the gyro is inclined upward from the horizontal plane by the angle .theta. as viewing from the west. As shown in FIG. 3, a gravitational acceleration g acts on the damping weight 7 of mass m so that a force of m.times.g acts on the damping weight 7 in the Vertical direction. In this case, let us consider that this force is divided into a component m g cos .theta. parallel to the vertical shafts 2, 2' and a component m g sin .theta. parallel to the spin axis. The component m g cos .theta. parallel to the vertical shafts 2, 2' acts only as a load on the vertical shaft ball bearings 4, 4', while the component m g sin .theta. parallel to the spin axis acts on the gyro as a torque multiplied with a distance r.sub.2 from the vertical shafts 2, 2' around the vertical shafts 2, 2'. Assuming that T.phi. represents the above torque, then the torque T.phi. is approximately given by the following equation: EQU T.phi.=.mu..multidot..theta.
where .mu.=m g r.sub.2.
That is, the damping weight 7 can be regarded as the apparatus which applies the vertical axes 2, 2' of the gyro with the torque proportional to the inclination of the gyro spin axis relative to the horizontal plane, and the north-seeking motion of the compass can be damped by the damping weight 7.
Further, a torque T.phi.1 generated during the ship's sailing is expressed by the following equation, considering the acceleration caused by ship's motion: ##EQU2##
FIG. 4 shows in block form a principle of the conventional gyro compass of FIG. 1, that is, the north-seeking motion of the conventional gyro compass in which an azimuthal error .phi. and an inclined angle .theta. from the due north of the north-seeking end of the gyro spin axis are assumed to be variables and which copes with their initial errors .phi..sub.O, .theta..sub.O are expressed by Laplace operator and transfer function in a block form. In FIG. 4, .OMEGA. represents earth rotation angular velocity, H angular momentum of gyro, .lambda. latitude of that spot, K north-seeking constant (ballistic constant), .mu. damping constant and S Laplace operator.
If now there is the azimuthal error .phi., then a component in which the azimuthal error .phi. is multiplied with a horizontal component .OMEGA. cos .lambda. 100 of the earth rotation velocity .OMEGA. acts on an element 101 around the horizontal axis of the gyro as an angular velocity input, thereby generating the gyro angle .theta. together with an initial inclined angle .theta..sub.O. The vertical ring 3 is similarly inclined by the inclination angle .theta. of the gyro spin axis, and the liquid ballistic 6 attached to the vertical ring 3 is also inclined, thereby the liquid 6-2 within the liquid ballistic 6 being moved in the lower reservoir, thereby a torque K.theta. being generated around the horizontal shaft of the gyro. This torque K.theta. is divided by the angular momentum H of the gyro and is then added with a vertical component .OMEGA. sin .lambda. of the earth rotation angular velocity, thereby being generated as an angular velocity input. This angular velocity input acts on a vertical shaft element 102 of the gyro, and this angular velocity input is added with the initial azimuth error .phi..sub.O to produce the azimuth error .phi., thereby closing the loop. This loop is what might be called a north-seeking loop of the gyro compass. Since two poles expressed by 1/S exist within this loop, this loop becomes an oscillating solution. On the other hand, a torque .mu..theta. is obtaibed by multiplying the gyro inclined angle .theta. with the damping constant .mu. and this torque .mu..theta. is divided by the angular momentum H so as to provide an angular velocity input. This angular velocity input is negatively fed back to the horizontal element 101 of the gyro so as to decrease the above inclined angle .theta., thereby the north-seeking motion of the north-seeking loop being damped. This latter loop is a damping loop.
In order to prevent an acceleration error from being caused in the gyro compass due to horizontal accelerations, such as increase and decrease of speed, turning or the like of the ship, the marine gyro compass is generally designed such that the north-seeking motion cycle is selected to be about 90 minutes (Schuler's condition). For this reason, it takes plenty of time for the gyro compass to be settled to the true north so as to be operable since the gyro compass has been energized. This time is what might be called a settle time.
In the ordinary ships, the above settle time does not raise a problem in their navigation substantially, however, this long settle time raises a problem in the ships for some special use.
Accordingly, Japanese Laid-Open Patent Publication No. 1-113611 describes a gyro compass having a fast settle apparatus which can reduce its settle time.
This conventional fast settle apparatus will now be described. In such a gyro compass which is comprised of a gyro case housing therein a gyro whose spin axis is kept substantially horizontal, a supporting apparatus for supporting the gyro case with three-axis freedom and having a function for outputting an inclined angle of the spin axis of the gyro relative to the horizontal plane and a function for applying a torque to the vertical axis of the gyro case in proportion to the input signal, the fast settle apparatus includes a control apparatus which is supplied with a signal corresponding to the inclined angle. In this control apparatus, the signal corresponding to the inclined angle is differentiated during a predetermined time since the gyro compass has been energized and a resultant differentiated signal is used as the above-mentioned input signal, thereby the settle time being reduced.
In the gyro compass having the above fast settle apparatus, if the latitude at which the gyro compass is located is changed, e.g., at a high latitude if the gyro compass is settled by operating the fast settle apparatus, then the above-mentioned north-seeking motion is placed in the so-called over-damping state due to the action of the torque generated around the vertical axis of the gyro by the damping constant .mu. of the damping loop. As a consequence, the settle time is increased so that, even if the gyro compass is provided with the fast settle apparatus, the settle time cannot be reduced in the high latitude as expected.
Further, in FIG. 5 which shows a schematic block diagram of the gyro compass according to the prior art, g represents the gravitational acceleration, R the earth radius, .OMEGA. the rotation angular velocity of earth, H the angular momentum of gyro, .lambda. the latitude at that spot, T.sub.O the time constant provided when the movement of the liquid surface of the ballistic 6 is approximated by the primary delay, K the north-seeking constant, .mu. the damping constant, .alpha..sub.N the acceleration acting on the north-south direction of the gyro case due to the ship's movement, V.sub.NS the north-south velocity of the ship and S the Laplace operator.
A sum of the gyro inclined angle .theta. and a value .alpha..sub.N /g, which results from dividing the north-south acceleration .alpha..sub.N by the gravitational acceleration g, acts on the primary delay transfer element 50 (time constant T.sub.O) provided by the liquid 6-2 of the ballistic 6 to form the liquid surface inclination .xi..
A precessional angular velocity ##EQU3## provided by multiplying .xi. with a value K/H (51), which results from dividing the north-seeking constant K by the angular momentum H of the gyro and which is generated around the vertical axis acts around the vertical axis of the gyro case 52 together with the vertical component .OMEGA. sin .lambda. of the earth rotation angular velocity .OMEGA. to produce the azimuthal movement around the vertical axis. Then, the azimuth error .phi. is generated. A value, which results from multiplying the azimuthal error .phi. with the horizontal component .OMEGA. cos .lambda. 53 of the earth rotation angular velocity .OMEGA., is input to a gyro element 54 around the horizontal axis of the gyro as the angular velocity input to thereby generate the gyro inclined angle .theta..
The above-mentioned portion is what might be called a north-seeking loop of the gyro compass, in which two poles expressed by 1/S exist within the loop, thereby generating the oscillation solution.
An angular velocity ##EQU4## which results from dividing by the gyro angular momentum H the torque ##EQU5## around the vertical axis in which ##EQU6## which results adding the gyro inclined angle .theta. with ##EQU7## is multiplied with the damping constant .mu. is input to a gyro element 54 around the horizontal axis together with the equivalent angular velocity V.sub.NS /R which results from dividing the north-south speed V.sub.NS of ship by the earth radius R, whereby the gyro inclined angle .theta. is reduced and the north-seeking movement is damped. Therefore, this loop is called a damping loop.
For the north-seeking loop, the north-south velocity V.sub.NS generates an azimuth error .phi..sub.V proportional to second of the latitude expressed by the following equation. ##EQU8## where C is the azimuth angle of the ship's heading.
FIG. 6 is a graph illustrating the movement of the gyro when the ship turns by 180.degree. at time t.sub.1 from the condition that the ship sails straight ahead on the course 0.degree. for a long time and the gyro compass is settled with velocity error .phi..sub.V1 at that time and then the ship sails straight ahead on the course 180.degree. from time t.sub.2. This fundamental influence of the gyro compass exerted by the acceleration can be reduced to the ordinary gyro compass.
An azimuth change .phi..sub.B generated by the acceleration between the time t.sub.1 and the time t.sub.2 is called as ballistic amount. A design method for making the azimuth change .phi..sub.B equal to the difference between the velocity errors before and after the acceleration acts is the important condition called the Schuler tuning in the gyro compass and corrects the influence of the acceleration in the form of velocity error (the north-seeking cycle of the gyro compass is extended to 1 to 1.5 hours due to this condition). That is, EQU .phi..sub.B =.phi..sub.V1 -.phi..sub.V2
The above-mentioned ballistic amount .phi..sub.B is the function of the velocity difference and the difference of the velocity error is also the function of the latitude as expressed in the above-mentioned equation. Therefore, strictly speaking, the condition in the above-mentioned equation is established only in particular latitude (referred to as a reference latitude). In other latitudes, the error .DELTA..phi. of FIG. 6 is generated immediately after the ship turns and then in accordance with the fundamental movement characteristics of the gyro compass, the gyro compass carries out the damping movement toward the velocity error .phi..sub.V2 provided immediately after the ship turns.
Instead of the above-mentioned damping weight 7, there is proposed a method in which the north-seeking movement of the gyro compass is carried out by, for example, the inclinometer or tilt meter for outputting the inclined angle of the spin axis of the gyro compass relative to the horizontal plane, an amplifier supplied with the output of the tilt meter, a torquer supplied with the output of the amplifer and so on. This method has the advantage such that the damping characteristic of the north-seeking movement can be arbitrarily corrected only by adjusting the gain of the amplifier.
The system utilizing the electrical torquer instead of the mechanical damping weight in order to obtain the above-mentioned damping effect considerably depends on accuracy of apparatus for detecting the above-mentioned reference inclined angle.
However, high efficiency and high accuracy requested for the inclined angle detecting apparatus of the gyro compass for multi-purpose except a part of war-use makes the gyro compass expensive.
As a result, in the gyro compass utilizing the inexpensive inclined angle detecting apparatus available on the market, an error occurs in the azimuth transmission angle of the gyro compass due to the error appearing in the inclined angle detecting apparatus.